Transparent photoreceptor overcoatings

ABSTRACT

Highly transparent charge injection enabling species for electrophotographic overcoatings include copper (I) compounds dispersed throughout the overcoating or complexed into a charge transport matrix. The overcoatings contain an insulating, film forming continuous phase having charge transport molecules and the copper (I) compounds.

BACKGROUND OF THE INVENTION

This invention relates to electrophotography, and more particularly, toan improved overcoated electrophotographic imaging member and method ofmaking the electrophotographic imaging member.

Generally, electrophotographic imaging processes involve the formationand development of electrostatic latent images on the imaging surface ofa photoconductive member. The photoconductive member is usually imagedby uniformly electrostatically charging the imaging surface in the dark,and exposing the member to a pattern of activating electromagneticradiation, such as light, which selectively dissipates the charge in theilluminated areas of the member to form an electrostatic latent image onthe imaging surface. The electrostatic latent image is then developedwith a developer composition containing toner particles which areattracted to the photoconductive member in image configuration. Theresulting toner image may be transferred to a suitable receiving membersuch as paper.

The imaging surface of many photoconductive members is sensitive towear, ambient fumes, scratches and deposits which adversely affect theelectrophotographic properties of the imaging member. Overcoating layershave been proposed to overcome the disadvantageous characteristics ofthese photoreceptors. However, many of the overcoating layers adverselyaffect electrophotographic performance of the electrophotographicimaging member.

One type of insulating electrophotographic imaging member has at leastone photoconductive layer and an overcoating layer comprising aninsulating, film forming continuous phase comprising charge transportmolecules and finely divided charge injection enabling particlesdispersed in the continuous phase.

Overcoatings for photoreceptors have been disclosed in U.S. Pat. No.4,515,882. These overcoatings comprise an insulating film formingcontinuous phase comprising charge transport molecules and finelydivided charge injection enabling particles dispersed in the continuousphase. The imaging members have at least one photoconductive layer andthe overcoating layer. Where desired, a barrier layer may be provided inthe device interposed between the photoconductive layer and theovercoating layer. The devices disclosed in U.S. Pat. No. 4,515,882 canbe employed in an electrophotographic imaging process in which the outerimaging surface of the overcoating layer is uniformly charged in thedark. A sufficient electric field is applied across theelectrophotographic imaging member to polarize the charge injectionenabling particles whereby the charge injection enabling particlesinject charge carriers into the continuous phase of the overcoatinglayer. The charge carriers are transported to and trapped at theinterface between the photoconductive layer and the overcoating layer,and opposite space charge in the overcoating layer is relaxed by chargeemission from the charge injection enabling particles to the imagingsurface. The overcoating layer is essentially electrically insulatingprior to deposition of the uniform electrostatic charge on the imagingsurface.

The mechanism by which charge passes through the overcoating to thephotoreceptive surface in known devices is believed to involve theelectric field, formed by corona charging of the electrophotographicdevice, instantly polarizing the charge injection enabling particles orspecies. Charge, for example, in the form of holes, is injected into thehole transport phase of the overcoating and is driven by the chargingfield to the interface between the overcoating and photoconductivelayer. The charge is stopped at the interface by a blocking layer orbecause there is no injection into the photoreceptor. The negative spacecharge in the bulk of the overcoating is relaxed by a charge emission.

However, overcoatings such as those disclosed in U.S. Pat. No. 4,515,882suffer from the disadvantage of high light absorption and scattering inthe coating due to pigment loading and particle size. Inorganic chargeinjection enabling particles mentioned in that patent include carbonblack, molybdenum disulfide, silicon, tin oxide, antimony oxide,chromium dioxide, zinc dioxide, titanium oxide, magnesium oxide,manganese dioxide, aluminum oxides, colloidal silica, graphite, tin,aluminum, nickel, steel, silver, gold, other metals and their oxides,sulfides, halides and other salt forms, etc. Such charge injectionenabling particles tend to reduce the photosensitivity of thephotoreceptor. For example, one weight percent of carbon black pigment,which is the prime effective charge injection enabling species currentlyin use, reduces light transmission to the photosensitive layer by about20%.

Electrophotographic devices have been proposed which include layers thatare electrically conducting and transparent. For example, U.S. Pat. No.3,505,131 discloses a method of preparing a highly transparent cuprousiodide conductive film. U.S. Pat. No. 3,677,816 discloses a method ofproducing transparent and electrically conducting coatings of copperiodide. These films are used as an electrode or ground in multielectrodeelectrostatic systems.

Copper iodide has also been used in electrophotography in protectivelayers, as disclosed in Japanese Unexamined Patent Application No.59-159 (1984). The disclosed protective layer comprises 10-60 weightpercent Cu iodide based on binder resin.

Another use of copper iodide in electrophotography is disclosed in U.S.Pat. No. 4,133,933. Cuprous iodide is provided in an electrosensitiverecording sheet, and is whitened by adding an alkaline substance forincreasing the resistance of the cuprous iodide and for increasing thecontrast of the recorded mask.

In the above-described devices, copper iodide is utilized primarily toachieve high electrical conductivity.

There continues to be a need for improved layers in electrophotographicimaging members which are highly transparent and which will protect theimaging member from wear, ambient fumes and the like.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided anelectrophotographic imaging member having at least one photoconductivelayer and an overcoating layer comprising an insulating, film formingcontinuous phase comprising charge transport molecules and highlytransparent charge injection enabling species. Copper (I) compounds suchas cuprous iodide are utilized as the charge injection enabling species.Where desired, a barrier layer may be interposed between thephotoconductive layer and the overcoating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention can be obtainedby reference to the accompanying drawing which shows a cross-sectionalview of a multilayer photoreceptor of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The electrophotographic imaging member of the present inventioncomprises an overcoating layer, preferably adjacent a photoconductivelayer. The overcoating layer comprises charge transport molecules andhighly transparent charge injection enabling particles in an insulatingfilm forming continuous phase. The overcoatings of the present inventionmay be used for negative and positive photoreceptors and are ofparticular interest for positive charging layered photoreceptors wherethe photoconductive charge generation and injection layer is on the topsurface, less than about one micron thick and subject to wear which, inthe absence of the present invention, would lead to short receptor life.

Any suitable insulating film forming binder having a very highdielectric strength and good electrically insulating properties may beused in the continuous charge transporting phase of the overcoating ofthe present invention. The binder itself may be a charge transportingmaterial or a material capable of holding transport molecules in solidsolution or as a molecular dispersion. A solid solution is defined as acomposition in which at least one component is dissolved in anothercomponent and which exists as a homogeneous solid phase. A moleculardispersion is defined as a composition in which particles of at leastone component are dispersed in another component, the dispersion of theparticles being on a molecular scale.

Typical film forming binder materials that are not charge transportingmaterials include thermoplastic and thermosetting resins such aspolycarbonates, polyesters, polyamides, polyurethanes, polystyrenes,polyarylethers, polyarylsulfones, polybutadienes, polysulfones,polyethersulfones, polyethylenes, polypropylenes, polyimides,polymethylpentenes, polyphenylene sulfides, polyvinylacetate,polysiloxanes, polyacrylates, polyvinylacetals, polyamides, aminoresins, phenylene oxide resins, terephthalic acid resins, epoxy resins,phenolic resins, polystyrene and acrylonitrile copolymers,polyvinylchloride, vinyl chloride and vinyl acetate copolymers, acrylatecopolymers, alkyd resins, cellulosic film formers, poly(amide-imide),styrene-butadiene copolymers, vinylidenechloride-vinylchloridecopolymers, vinyl acetate-vinylidenechloride copolymers, styrene alkydresins, fluorocarbon resins, and the like.

Typical film forming binder materials that have charge transportcapabilities are substantially non-absorbing in the spectral region ofintended use, but are active in that they are capable of transportingcharge carriers injected by the charge injection enabling particles inan applied electric field. The charge transport binder may be a holetransport film forming polymer or an electron transport film formingpolymer. Charge transporting film forming polymers are well known in theart and include those enumerated in U.S. Pat. No. 4,515,882. Othertransport polymers include arylamine compounds disclosed in U.S. Pat.Nos. 4,806,443, 4,806,444 and 4,818,650, as well as polysilylenesdisclosed in U.S. Pat. Nos. 4,618,551, 4,774,159, 4,772,525 and4,758,488.

The film forming binder should have an electrical resistivity of atleast about 10¹³ ohm-cm. It should be capable of forming a continuousfilm and be substantially transparent to activating radiation to whichthe underlying photoconductive layer is sensitive. In other words, thetransmitted activating radiation should be capable of generating chargecarriers, i.e. electron-hole pairs, in the underlying photoconductivelayer or layers. A transparency range of between about 35 percent andabout 100 percent can provide satisfactory results depending upon thespecific photoreceptors utilized. A transparency of at least about 50percent is preferred for greater speed with optimum speeds beingachieved at a transparency of at least greater than 90 percent.Transparency is meant to refer to the property of permitting the passageof radiations in the spectral region at which the underlyingphotoconductive layer or layers are sensitive.

Any suitable charge transport molecule capable of acting as a filmforming binder or which is soluble or dispersable on a molecular scalein a film forming binder may be utilized in the continuous phase of theovercoating of this invention. The charge transport molecules should becapable of transporting charge carriers injected by the charge injectionenabling particles in an applied electric field. The charge transportmolecules may be hole transport molecules or electron transportmolecules. Where the charge transport molecule is capable of acting as afilm forming binder as indicated above, it may be employed, if desired,to function as both an insulating binder for the charge injectionenabling particles and as the continuous charge transporting phasewithout the necessity of incorporating a different charge transportmolecule in solid solution or as a molecular dispersion therein.

Such charge transporting materials are well known in the art. Diamines,pyrazolines, substituted fluorenes, oxidiazoles, hydrazones,tri-substituted methanes, transparent organic non-polymeric transportmaterials, and the like, as disclosed in U.S. Pat. No. 4,515,882, areexamples of well known charge transporting materials.

When the charge transport molecules are combined with an insulating filmforming binder, the amount of charge transport molecule which is usedmay vary depending upon the particular charge transport material, itscompatibility with (e.g. solubility in) the continuous insulating filmforming binder phase of the overcoating layer, and the like.Satisfactory results have been obtained using the proportions normallyused to form the charge transport medium of photoreceptors containing acharge transport component and a charge generating component.

When the overcoating layers are prepared with only insulating filmforming binder and charge transport molecules in solid solution ormolecular dispersion in the film forming binder, the overcoating layerremains insulating after charging until at least the image exposurestep. However, when sufficient charge injection enabling particles aredispersed in an overcoating layer containing an insulating film formingcontinuous phase capable of transporting charge carriers, theovercoating layer acquires the capability of being an insulator until asufficient electric field is applied to polarize the charge injectionenabling particles. Then, the charge injection enabling particles injectcharge carriers into the continuous phase of the overcoating layer. Thecharge carriers are transported to and trapped at the interface betweenthe underlying photoconductive layer and the overcoating layer. Oppositespace charge in the overcoating layer is relaxed by charge emission fromthe charge injection enabling particles to the outer imaging surface ofthe overcoating.

The charge injection enabling particles of the present invention arecomprised of a copper (I) compound. Copper (I) compounds have desirableproperties in electrophotographic applications; for example, copper (I)compounds have desirable electrical properties, which properties areuseful in electrically conducting ground planes and other conductiveelements of a photoreceptor. Copper (I) compounds which can be used inthe invention include cuprous iodide, cuprous bromide and cuprouschloride. A preferred copper (I) compound is cuprous iodide.

Cuprous iodide is an electrical conductor in bulk and film form, and iscolorless and therefore highly transparent in the visible region byvirtue of the d¹⁰ electronic configuration of the Cu⁺ ion, the colorlessI⁻ ion and the lack of charge transfer bands in the visible region. Thetransparent properties of cuprous iodide are desirable in the presentinvention. High pigment loadings of CuI are possible with little or nolight absorption to reduce the photosensitivity of the photoreceptor.

The charge injection properties of copper (I) compounds have notpreviously been studied in a polymer transport matrix. Furthermore,relative to other transparent conductors such as Cd₂ SnO₄ or the tinoxides containing In or Sb, cuprous iodide is generally considered to beless environmentally hazardous.

Cuprous iodide can adsorb surface moisture, oxygen, iodine and otherspecies that may be used to control the charge injecting properties ofthe material in the matrix. The presence or absence of moisture, forexample, may be controlled by the method and length of time of drying.The presence or absence of iodine may also be controlled. For example,in the reaction for the formation of CuI

    2CuSO.sub.4 +4KI+2Na.sub.2 S.sub.2 O.sub.3 →2CuI+2K.sub.2 SO.sub.4 +Na.sub.2 S.sub.4 O.sub.6 +2NaI,

the copper (II) iodide formed initially by the combination of copper(II) ion and iodide ion in aqueous solution decomposes almostimmediately by a redox reaction to yield copper (I) iodide and freeiodine. The amount of free iodine in the sample may be controlled oreliminated by varying the amount of thiosulfate. The absorption ofiodine in the sample may be desirable since its absorption increases theconductivity of the copper (I) iodide.

Light also has an effect on the surface properties of CuI, although itssensitivity to light is much less than that of CuBr which is much lessthan that of CuCl. Exposure to light will increase the conductivity ofcopper (I) iodide. This convenient type of controlling surfaceproperties is not known for other transparent conductors such as SnO₂,doped SnO₂ or Cd₂ SnO₄ l .

Generally, the overcoating layer should contain at least about 0.1percent by weight of the charge injection enabling particles based onthe total weight of the overcoating layer. At lower concentrations, anoticeable residual charge tends to form, which can be compensatedduring development by applying an electric bias as is known in the art.The upper limit for the amount of the charge injection enablingparticles to be used depends upon the relative quantity of charge flowdesired through the overcoating layer, but should be less than thatwhich would reduce the transparency of the overcoating to a value lessthan about 35 percent and which would render the overcoating tooconductive.

The amount of charge injection enabling particles which can be loaded inthe overcoating layer of the present invention may range from about 1 toabout 25 weight percent based on the total weight of the overcoatinglayer. The particular loading of charge injection enabling particleswill depend on the desired percent transmission, desired conductivity,the binding capability of the resin, the desired mechanical propertiesof the imaging member, e.g., flexibility, and the residual voltage onthe photoreceptor. With copper (I) compounds such as cuprous iodide, theloading may be from about 1 to about 25 weight percent based on weightof the total weight of the overcoating layer. A particularly preferredloading of copper iodide is 3 to 20 weight percent and most preferablyabout 10 to 20 weight percent. With such loadings, transparent layershaving a resistivity greater than about 10¹¹ ohms-cm can be obtained.

The particle size of the charge injection enabling particles should beless than about 25 microns, preferably less than about 1 micron, and formolecular dispersions less than the wavelength of light utilized toexpose the underlying photoconductive layers. In other words, theparticle size should be sufficient to maintain the overcoating layersubstantially transparent to the wavelength of light to which theunderlying photoconductive layer or layers are sensitive. A particlesize between about 100 Angstroms and about 500 Angstroms has been foundmost suitable for light sources having a wavelength greater than about4,000 Angstroms. The particle size of the charge injection enablingparticles of the present invention may be controlled by the preparativeroute used to make the copper (I) compounds and their dispersions.

In addition to the advantages already mentioned, cuprous iodide andother copper (I) compounds have the added advantage that they can formdonor-acceptor complexes, for example, with amines or ammonia byinteraction between the nitrogen lone-pair of electrons and the Cu(I)ion. Thus, the potential exists for weak complexes to form in solutionbetween CuI and material comprising the charge transport layer foradditionally transparent overcoatings.

For example, a charge transport layer may comprise a charge transportcompound having the general formula: ##STR1## wherein X is selected fromthe group consisting of an alkyl group, having from 1 to about 4 carbonatoms, and chlorine. This particular compound will hereinafter bereferred to as TAA. Weak complexes can form in solution between CuI andTAA. A surface of adsorbed TAA on CuI in the matrix may also beenvisioned, establishing an intimate electronic contact between theinjecting and transport species. Transparency may be increased if thecharge transporting molecules promote wetting of the matrix to CuI whichwill reduce voids at the interface to enhance the index of refractiongradient across the interface.

Other charge transport matrix materials which may be molecularlycomplexed with CuI include phosphine derivatives of TAA andpolysilylenes disclosed in U.S. Pat. Nos. 4,618,551, 4,774,159,4,772,525 and 4,758,488.

The components of the overcoating layer may be mixed together byconventional means. Typical mixing means include stirring rods,ultrasonic vibrators, magnetic stirrers, paint shakers, sand mills, rollpebble mills, sonic mixers, melt mixing devices and the like. It isimportant, however, that if the insulating film forming binder is adifferent material than the charge transport molecules, the chargetransport molecules must either dissolve in the insulating film formingbinder or be capable of being molecularly dispersed in the insulatingfilm forming binder. A solvent or solvent mixture for the film formingbinder and charge transport molecules may be utilized if desired.Preferably, the solvent or solvent mixture should dissolve both theinsulating film forming binder and the charge transport molecules. Thesolvent selected should not adversely affect the underlyingphotoreceptor. For example, the solvent selected should not dissolve orcrystallize the underlying photoreceptor.

The overcoating mixture may be applied to the photoconductive member orto a blocking layer, if a blocking layer is utilized. The overcoatingmixture may be applied by known techniques. Typical coating techniquesinclude all spraying techniques, draw bar coating, dip coating, gravurecoating, silk screening, air knife coating, reverse roll coating,extrusion techniques and the like. Conventional drying or curingtechniques may be utilized to dry the overcoating. The drying or curingconditions should be selected to avoid damaging the underlyingphotoreceptor. For example, the overcoating drying temperatures shouldnot cause crystallization of amorphous selenium when an amorphousselenium photoreceptor is used.

The thickness of the overcoating layer after drying or curing may bepreferably between about 1 micron and about 15 microns. Generally,overcoating thicknesses less than about 1 micron fail to providesufficient protection for the underlying photoreceptor. Greaterprotection is provided by an overcoating thickness of at least about 3microns. Resolution of the final toner image begins to degrade when theovercoating thickness exceeds about 15 microns. Clearer image resolutionis obtained with an overcoating thickness less than about 8 microns.Thus, an overcoating thickness of between 3 microns and about 8 micronsis preferred for optimum protection and image resolution.

The final dried or cured overcoating should be substantially insulatingprior to charging. Satisfactory results may be achieved when the finalovercoating has a resistivity of at least about 10¹¹ ohm-cm, preferably10¹³ ohm-cm, at fields low enough essentially to eliminate injectionfrom the charge injection enabling particles into the transportmolecule. The overcoating is substantially electrically insulating inthe dark. The charge injection enabling particles will therefore notpolarize in less than about 10⁻¹² second and inject charge carriers intothe continuous charge transporting phase in less than about 10microseconds when an applied electric field less than about 5 volts permicron is applied across the imaging member from the conductivesubstrate to the outer surface of the overcoating.

The final dried or cured overcoating of the present invention issubstantially non-absorbing in the spectral region at which theunderlying photoconductive layer or layers are sensitive. The expression"substantially non-absorbing" is defined as a transparency of betweenabout 35 percent and about 90 percent in the spectral region at whichthe underlying photoconductive layer or layers are sensitive. Atransparency of at least about 50 percent in the spectral region atwhich the underlying photoconductive layer or layers are sensitive ispreferred for a balance of electrical and optical properties in thecoating speed with optimum speeds being achieved at a transparency of atleast greater than 90 percent.

The overcoatings of the present invention may also reduce emission oftoxic Se, Te and As particles generated from alloy photoreceptors ofxerographic machines used in making copies. They may also inhibitcrystallization of Se/Te alloys by chemical exposure to, e.g., mercuryvapor in dental offices. Further, the overcoatings prevent extraction ofcharge transport molecules from layered photoreceptors in use withliquid developers.

Any suitable electrophotoconductive member may be overcoated with theovercoating layer of this invention. Generally, anelectrophotoconductive member comprises one or more photoconductivelayers on a supporting substrate.

The substrate may be opaque or substantially transparent and maycomprise numerous suitable materials having the required mechanicalproperties. Accordingly, this substrate may comprise a layer of anon-conductive or conductive material such as an inorganic or an organiccomposition. If the substrate comprises non-conductive material, it isusually coated with a conductive composition. As insulatingnon-conducting materials there may be employed various resins known forthis purpose including polyesters, polycarbonates, polyamides,polyurethanes, and the like. The insulating or conductive substrate maybe flexible or rigid and may have any number of many differentconfigurations such as, for example, a plate, a cylindrical drum, ascroll, an endless flexible belt, and the like. Preferably, theinsulating substrate is in the form of an endless flexible belt and iscomprised of a commercially available polyethylene terephthalatepolyester known as Mylar available from E. I. du Pont de Nemours & Co.

The thickness of the substrate layer depends on numerous factors,including economical considerations, and thus this layer may be ofsubstantial thickness, for example, over 200 microns, or of minimumthickness less than 50 microns, provided there are no adverse affects onthe final photoconductive device. In one embodiment, the thickness ofthis layer ranges from about 65 microns to about 150 microns, andpreferably from about 75 microns to about 125 microns.

A conductive layer or ground plane which may comprise the entire supportor be present as a coating on a non-conductive layer may comprise anysuitable material including, for example, aluminum, titanium, nickel,chromium, brass, gold, stainless steel, carbon black, graphite and thelike. The conductive layer may vary in thickness over substantially wideranges depending on the desired use of the electrophotoconductivemember. Accordingly, the conductive layer can generally range inthickness of from about 50 Angstroms to many centimeters. When aflexible photoresponsive imaging device is desired, the thickness may bebetween about 100 Angstroms to about 750 Angstroms, and more preferablyfrom about 100 Angstroms to about 200 Angstroms.

Any suitable photoconductive layer or layers may be overcoated with theovercoating layer of this invention. The photoconductive layer or layersmay be inorganic or organic. Typical inorganic photoconductive materialsinclude well known materials such as amorphous selenium, seleniumalloys, halogen-doped selenium alloys such as selenium-tellurium,selenium-tellurium-arsenic, selenium-arsenic, and the like, cadmiumsulfoselenide, cadmium selenide, cadmium sulfide, zinc oxide, titaniumdioxide and the like. Typical organic photoconductors includephthalocyanines, quinacridones, pyrazolones,polyvinylcarbazole-2,4,7-trinitrofluorenone, anthracene and the like.Many organic photoconductors may be used as particles dispersed in aresin binder.

Any suitable multilayer photoconductors may also be employed with theovercoating layer of this invention. The multilayer photoconductorscomprise at least two electrically operative layers, a photogeneratingor charge generating layer and a charge transport layer. Examples ofphotogenerating layers include trigonal selenium, various phthalocyaninepigments such as the X-form of metal free phthalocyanine described inU.S. Pat. No. 3,357,989, metal phthalocyanines such as copperphthalocyanine, quinacridones available from Du Pont under the tradenameMonastral Red, Monastral violet and Monastral Red Y, substituted2,4-diamino-triazines disclosed in U.S. Pat. No. 3,442,781, polynucleararomatic quinones available from Allied Chemical Corporation under thetradename Indofast Double Scarlet, Indofast Violet Lake B, IndofastBrilliant Scarlet and Indofast Orange. Examples of photosensitivemembers having at least two electrically operative layers include thecharge generating layer and diamine containing transport layer membersdisclosed in U.S. Pat. No. 4,254,990; dyestuff generator layer andoxadiazole, pyrazalone, imidazole, bromopyrene, nitrofluorene andnitronaphthalimide derivative containing charge transport layer membersdisclosed in U.S. Pat. No. 3,895,944; generator layer and hydrazonecontaining charge transport layer members disclosed in U.S. Pat. No.4,150,987; generator layer and a tri-aryl pyrazoline compound containingcharge transport layer members disclosed in U.S. Pat. No. 3,837,851; andthe like.

A preferred multilayered photoconductor comprises a charge generatinglayer comprising a layer of photoconductive material and a contiguouscharge transport layer of a polycarbonate resin material having amolecular weight of from about 20,000 to about 120,000 having dispersedtherein from about 25 to about 75 percent by weight of one or morecompounds having the general formula: ##STR2## wherein X is selectedfrom the group consisting of an alkyl group having from 1 to about 4carbon atoms and chlorine. The photoconductive layer exhibits thecapability of photogeneration of holes and injection of the holes. Thecharge transport layer is substantially non-absorbing in the spectralregion at which the photoconductive layer generates and injectsphotogenerated holes from the photoconductive layer and transports theholes through the charge transport layer. Other examples of chargetransport layers capable of supporting the injection of photogeneratedholes of a charge generating layer and transporting the holes throughthe charge transport layer include triphenylmethane,bis(4-diethylamine-2-methylphenyl) phenylmethane;4'-4"-bis(diethylamino)-2',2"-dimethyltriphenyl methane and the likedispersed in an inactive resin binder.

Numerous inactive resin materials may be employed in the chargetransport layer including those described, for example, in U.S. Pat. No.3,121,006. The resinous binder for the charge transport layer may beidentical to the resinous binder material employed in the chargegenerating layer. Typical organic resinous binders include thermoplasticand thermosetting resins such as polycarbonates, polyesters, polyamides,polyurethanes, polystyrenes, polyarylethers, polyarylsulfones,polybutadienes, polysulfones polyethersulfones, polyethylenes,polypropylenes, polyimides, polymethylpentenes, polyphenylene sulfides,polyvinyl acetate, polysiloxanes, polyacrylates, polyvinyl acetals,amino resins, phenylene oxide resins, terephthalic acid resins, epoxyresins, phenolic resins, polystyrene and acrylonitrile copolymers,polyvinylchloride, vinylchloride and vinyl acetate copolymers, acrylatecopolymers, alkyd resins, cellulosic film formers, poly(amide-imide),styrene-butadiene copolymers, vinylidenechloride-vinylchloridecopolymers, vinylacetate-vinylidenechloride copolymers, styrenealkydresins, and the like. These polymers may be block, random or alternatingcopolymers. Excellent results may be achieved with a resinous bindermaterial comprising a poly(hydroxyether) material selected from thegroup consisting of those of the following formulas: ##STR3## wherein Xand Y are independently selected from the group consisting of aliphaticgroups and aromatic groups, Z is hydrogen, an aliphatic group or anaromatic group, and n is a number of from about 50 to about 200.

These poly(hydroxyethers), some of which are commercially available fromUnion Carbide Corporation, are generally described in the literature asphenoxy resins or epoxy resins.

Examples of aliphatic groups for the poly(hydroxyethers) include thosecontaining from about 1 carbon atom to about 30 carbon atoms, such asmethyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, decyl, pentadecyl,eicodecyl, and the like. Preferred aliphatic groups include alkyl groupscontaining from about 1 carbon atom to about 5 carbon atoms, such asmethyl, ethyl, propyl, and butyl. Illustrative examples of aromaticgroups include those containing from about 6 carbon atoms to about 25carbon atoms, such as phenyl, naphthyl, anthryl, and the like, withphenyl being preferred. The aliphatic and aromatic groups can besubstituted with various known substitutents, including, for example,alkyl, halogen, nitro, sulfo and the like.

Examples of the Z substituent include hydrogen as well as aliphaticaromatic, substituted aliphatic and substituted aromatic groups asdefined herein. Furthermore Z can be selected from carboxyl, carbonate,and other similar groups, resulting in for example, the correspondingesters, and carbonates of the poly(hydroxyethers).

Preferred poly(hydroxyethers) include those wherein X and Y are alkylgroups, such as methyl, Z is hydrogen or a carbonate group, an n is anumber ranging from about 75 to about 100. Specific preferredpoly(hydroxyethers) include Bakelite, phenoxy resins PKHH, commerciallyavailable from Union Carbide Corporation and resulting from the reactionof 2,2-bis(4-hydroxyphenylpropane), or bisphenol A, withepichlorohydrin, an epoxy resin, Araldite R 6097, commercially availablefrom CIBA, the phenylcarbonate of the poly(hydroxyethers) wherein Z is acarbonate grouping, which material is commercially available from AlliedChemical Corporation, as well as poly(hydroxyethers) derived fromdichloro bisphenol A, tetrachloro bisphenol A, tetrabromo bisphenol A,bisphenol F, bisphenol ACP, bisphenol L, bisphenol V, bisphenol S, andthe like.

The photogenerating layer containing photoconductive compositions and/orpigments and the resinous binder material generally ranges in thicknessfrom about 0.1 micron to about 5.0 microns, and preferably has athickness of from 0.3 micron to about 1 micron. Thicknesses outsidethese ranges can be selected providing the objectives of the presentinvention are achieved.

The photogenerating composition or pigment is present in thepoly(hydroxyethers) resinous binder composition in various amounts.Generally from about 10 percent by volume to about 60 percent by volumeof the photogenerating pigment is dispersed in about 40 percent byvolume to about 90 percent by volume of the poly(hydroxyether) binder.Preferably from about 20 percent by volume to about 30 percent by volumeof the photogenerating pigment is dispersed in about 70 percent byvolume to about 80 percent by volume of the poly(hydroxyether) bindercomposition. In one embodiment about 25 percent by volume of thephotogenerating pigment is dispersed in about 75 percent by volume ofthe polyhydroxyether binder composition.

Other typical photoconductive layers include amorphous or alloys ofselenium such as selenium-arsenic, selenium-tellurium-arsenic,selenium-tellurium, selenium-arsenic-antimony, halogen doped seleniumalloys, cadmium sulfide and the like.

Generally, the thickness of the transport layer is between about 5 toabout 100 microns, but thicknesses outside this range can also be used.The charge transport layer should be an insulator to the extent that theelectrostatic charge placed on the charge transport layer is notconducted in the absence of illumination at a rate sufficient to preventformation and retention of an electrostatic latent image thereon. Ingeneral, the ratio of the thickness of the charge transport layer to thecharge generator layer is preferably maintained from about 2:1 to 200:1,and in some instances as great as 400:1.

The following are examples of overcoatings prepared with an insulatingfilm forming binder polymer, Merlon M-50F polycarbonate, available fromMobay Chemical Company, an active hole transporting material TAA and acharge injecting enabling particulate material cuprous iodide. Theexamples are intended to be illustrative only. The invention is notintended to be limited to the materials, conditions, process parametersand the like recited herein.

Comparative Example 1

The solution which was used for the spray application of the overcoatingconsisted of 16.3 gms of Merlon M-50F, 11.2 gms of TAA (40 percentweight), 660 gms of methylene chloride and 440 gms of 1,1,2trichloroethane. This solution did not contain the charge injectionenabling particles. It was applied by spray coating to a brush grainedaluminum plate and clear Mylar film. The coating was applied with aconventional automatic spray gun Model 21 manufactured by the BinksManufacturing Co. of Franklin Park, Ill. The coating was dried at 110°C. for 30 minutes and had a measured thickness of 4 microns. The visiblelight transmittance of the overcoating as measured on the clear Mylarsubstrate was 99.9 percent. The overcoating on the aluminum plate wasevaluated for charge injection by corona charging with a potential of+5000 and -5000 volts applied to the corotron wire. The charge on thesurface of the overcoating was measured with a conventionalelectrostatic voltmeter. The charge and measure cycle was repeatedseveral times to determine the stability of the charge on the surface ofthe overcoating. The results of these charge and measure cycles are asfollows.

    ______________________________________                                        Cycle                                                                         ______________________________________                                        Corotron Voltage                                                                           +5000 volts   -5000 volts                                        Surface Potential                                                             1            +56 volts     -248 volts                                         2            +136 volts    -264 volts                                         3            +168          -280 volts                                         ______________________________________                                    

EXAMPLE II

The procedure described in Example I was repeated except that thesolution for the spray application of the overcoating consisted of 16.3gms of Merlon M-50F, 11.2 gms of TAA (40 percent weight), 0.275 gms ofcuprous iodide (1 percent weight), 660 gms of methylene chloride and 440gms of 1,1,2 trichloroethane. The visible light transmittance of theovercoating was 99.9 percent. The results of the charge and measurecycles are as follows.

    ______________________________________                                        Cycle                                                                         ______________________________________                                        Corotron Voltage                                                                           +5000 volts   -5000 volts                                        Surface Potential                                                             1            +50 volts     -18 volts                                          2            +80 volts     -20 volts                                          3            +88           -24 volts                                          ______________________________________                                    

EXAMPLE III

The procedure described in Example I was repeated except that thesolution for the spray application of the overcoating consisted of 16.3gms of Merlon M-50F, 11.2 gms of TAA (40 percent weight), 1.375 gms ofcuprous iodide (5 percent weight), 660 gms of methylene chloride and 440gms of 1,1,2 trichloroethane. The visible light transmittance of theovercoating was 97.7 percent. The results of the charge and measurecycles are as follows.

    ______________________________________                                        Cycle                                                                         ______________________________________                                        Corotron Voltage                                                                           +5000 volts   -5000 volts                                        Surface Potential                                                             1            +24 volts     -6 volts                                           2            +32 volts     -8 volts                                           3            +32 volts     -8 volts                                           ______________________________________                                    

EXAMPLE IV

The procedure described in Example I was repeated except that thesolution for the spray application of the overcoating consisted of 16.3gms of Merlon M-50F, 11.2 gms of TAA (40 percent weight), 2.75 ofcuprous iodide (10 percent weight), gms of methylene chloride and 440gms of 1,1,2 trichloroethane. The visible light transmittance of theovercoating was 93.0 percent. The results of the charge and measurecycles are as follows.

    ______________________________________                                        Cycle                                                                         ______________________________________                                        Corotron Voltage                                                                           +5000 volts   -5000 volts                                        Surface Potential                                                             1            +8 volts      -2 volts                                           2            +14 volts     -2 volts                                           3            +16 volts     -4 volts                                           ______________________________________                                    

EXAMPLE V

The procedure described in Example I was repeated except that thesolution for the spray application of the overcoating consisted of 16.3gms of Merlon M-50F, 11.2 gms of TAA (40 percent weight), 4.125 gms ofcuprous iodide (15 percent weight), 660 gms of methylene chloride and440 gms of 1,1,2 trichloroethane. The visible light transmittance of theovercoating was 91.2 percent. The results of the charge and measurecycles are as follows.

    ______________________________________                                        Cycle                                                                         ______________________________________                                        Corotron Voltage                                                                           +5000 volts   -5000 volts                                        Surface Potential                                                             1            +8 volts      -2 volts                                           2            +16 volts     -2 volts                                           3            +16 volts     -4 volts                                           ______________________________________                                    

These results indicate that, without the charge injection enablingparticles, the 4 microns thick insulating film forming binder and chargetransport molecule layer charges to an unacceptable high voltage level.This level is reduced as larger amounts of the charge injection enablingparticles are introduced into the insulating film forming binder andcharge transport molecules. This indicates that cuprous iodide is aneffective charge injection enabling particulate material that injectscharge carriers into the continuous phase of the overcoating layer. Thecharge carriers are transported through the overcoating layer and to theconductive substrate where they combine with the opposite polaritycharge. Opposite space charge in the overcoating layer is relaxed bycharge emission from the charge injection enabling particles to theouter imaging surface of the overcoating.

EXAMPLE VI

The solutions prepared as described in Examples II, III, IV and V werespray coated onto organic photoreceptors which had a ground plane 1, acharge transport layer 2 and a charge generating layer 3. An electricalcharge blocking layer 4 was applied to the organic photoreceptor of theFigure prior to the application of the overcoating 5 to trap the chargecarriers which are produced by the overcoating during the application ofthe electric charge field to the overcoated photoreceptor. Theelectrical charge blocking layer consisted of about 1.0 micron of a oneto one weight ratio of zirconium acetylacetonate in Butvar B-72 from theMonsanto Polymers and Petrochemicals Co. of St. Louis, Mo. The coatingwas applied using the spray coating equipment described in Example I.The coating was dried at 110° C. for 30 minutes. The overcoating wasapplied to the organic photoreceptor with the electrical charge blockinglayer by use of the spray coating equipment described in Example I. Aphotoreceptor with the electrical charge blocking layer was spray coatedwith each of the overcoatings of Examples II, III, IV and V for printtesting and another was half coated for electrical cycling measurements.The overcoated photoreceptors were dried at 110° C. for 30 minutes.

The electrical measurements were made in a cycling scanner at arotational rate for the photoreceptor of 24 revolutions per minute. Thecharging was done at a constant current of 3.6 microamperes and a Xerox4045 machine erase lamp was used to discharge the photoreceptor beforerecharging. The voltage on the photoreceptor was measured at 0.20 and1.12 seconds after charging and after exposure to the erase lamp. Thedifference between the voltage measured at 0.20 and 1.12 seconds aftercharging divided by the difference in the measurement time correspondsto the dark decay of the voltage on the photoreceptor. Thephotoreceptors which were overcoated with the overcoating materials thathad less than 5 percent weight of cuprous iodide showed a widecircumferential variation in the initial voltage measured at 0.20seconds after charging. The initial, residual and dark decay voltagesdecreased with increasing loading of the cuprous iodide in theovercoating. The largest changes occurred for loadings of from 0 to 5weight percent of cuprous iodide in the overcoating. The initial voltagedecreased from 1200 volts to 740 volts indicating that the overcoatingwas effective in enabling injection and the charge was trapped at theinterface of the photoreceptor.

The initial voltage on the photoreceptor was the same for the overcoatedand unovercoated sides when the overcoating contained 15 weight percentof cuprous iodide. Cycling of the photoreceptor resulted in asignificant increase in the dark decay to 100 volts for the unovercoatedside as it degraded under the action of the corona from the chargingcorotron. There was no significant change in the initial voltage anddark decay for the overcoated side of the photoreceptor. The residualvoltage on the overcoated side of the photoreceptor increased to 75volts after 200 cycles while that on the unovercoated side stabilized at16 volts. The overcoating that contained 15 weight percent of cuprousiodide had the lowest residual voltage and best cycling characteristics.

Print tests illustrated that the photoreceptor with the overcoating thatcontained 15 weight percent of cuprous iodide gave good quality tonerdeveloped line copy as compared to the unovercoated photoreceptor. Noblurring of the developed image was observed although there was a slightgraininess to the toner developed image area. There was no difference inthe background quality for the overcoated versus the unovercoatedphotoreceptor. Continuous toner developed imaging of the overcoatedphotoreceptor was done and 4500 prints were obtained. The unovercoatedphotoreceptor failed after 2000 prints.

While the present invention has been described in detail with particularreference to preferred embodiments thereof, it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention as described herein above and as defined in theappended claims.

What is claimed is:
 1. An electrophotographic imaging member, comprisinga substantially transparent layer comprising charge transport moleculesand 3 to 25weight percent based on the total weight of said transparentlayer charge injection enabling species of a copper (I) compound,wherein said layer is prepared from a molecular dispersion of saidcopper (I) compound in a film forming continuous phase.
 2. Theelectrophotographic imaging member according to claim 1, wherein saidcopper (I) compound is cuprous halide.
 3. The electrophotographicimaging member according to claim 1, wherein said copper (I) compound iscuprous iodide.
 4. The electrophotographic imaging member according toclaim 1, wherein said charge injection enabling species are particles.5. The electrophotographic imaging member of claim 1, wherein said filmforming continuous phase is insulating.
 6. The electrophotographicimaging member of claim 1, wherein said transparent layer is aninsulating overcoating layer.
 7. The electrophotographic imaging memberof claim 1, wherein said transparent layer has a transparency of atleast about 35%.
 8. The electrophotographic imaging member of claim 1,wherein said transparent layer has a transparency greater than about90%.
 9. The electrophotographic imaging member of claim 1, wherein saidtransparent layer comprises about 10 to about 20 weight percent of saidcharge injection enabling species based on weight of said transparentlayer.
 10. The electrophotographic imaging member of claim 1, whereinsaid transparent layer comprises about 10 to about 20 weight percent ofcuprous iodide based on weight of said transparent layer.
 11. Theelectrophotographic imaging member of claim 1, wherein said transparentlayer has a resistivity greater than about 10¹¹ ohm-cm.
 12. Theelectrophotographic imaging member of claim 1, wherein said chargeinjection enabling species are molecularly complexed with said chargetransport molecules.
 13. An electrophotographic imaging member,comprising at least one photoconductive layer and an overcoating layercomprising a film forming continuous phase comprising charge transportmolecules and 3 to 25 weight percent based on the total weight of saidtransparent layer charge injection enabling species of a copper (I)halide compound, wherein said overcoating layer is prepared from amolecular dispersion of said copper (I) halide compound in saidcontinuous phase.
 14. The electrophotographic imaging member of claim13, wherein said copper (I) halide compound is cuprous iodide.
 15. Theelectrophotographic imaging member of claim 13, wherein said filmforming continuous phase is insulating.
 16. The electrophotographicimaging member of claim 13, wherein said charge transport moleculescomprise at least one compound having the formula: ##STR4## wherein X isselected from the group consisting of an alkyl group having from 1 toabout 4 carbon atoms and chlorine.
 17. The electrophotographic imagingmember of claim 13, wherein said charge transport molecules aremolecularly complexed with said copper (I) halide compound.
 18. Theelctrophotographic imaging member of claim 13, wherein said overcoatinglayer has a transparency of at least about 35%.
 19. Theelectrophotographic imaging member of claim 13, wherein said overcoatinglayer has a transparency of at least about 90%.
 20. Theelectrophotographic imaging member of claim 13, wherein said overcoatinglayer comprises about 10 to about 20 percent of cuprous iodide based onweight of said transparent layer.
 21. The electrophotographic imagingmember of claim 13, wherein said overcoating layer has a resistivitygreater than about 10¹¹ ohm-cm.